ABSTRACT
One of the major functions of skin is the provision of a protective barrier defending the body against noxious agents in the environment. The barrier has to be maintained by continued self renewal and repair of breaches caused by injury. The physical properties of stratum corneum limit the passage of water, electrolytes and many chemical substances. The epidermis functions as a physical barrier against invasion by microbes. However, the skin is more than a passive barricade-it has to play an active role, responding to perturbations, whether physical or chemical, by actively recruiting inflammatory mechanisms which can include specific components of the immune system. The involvement of the skin in immune defence is particularly important in relation to microbes that penetrate into the epidermis, such as viruses, fungi and parasites, including the scabies mite. In responding to chemical perturbations the skin cannot distinguish between microbial products and other environmentally derived substances. If the substance is immunogenic the immune system is activated and the response evoked by chemicals/antigens from microbes may be indistinguishable from that induced by non-microbial haptens-so-called contact sensitizers. The result is a delayed-type hypersensitivity characterized by the infiltration of CD4+ T lymphocytes into both dermis and epidermis. Experimental contact hypersensitivity has been used to elucidate mechanisms involved in the immune response both regarding the induction of sensitization, the afferent limb, and also the expression of the immune response in the efferent limb.
